A variety of different electrode structures are used in the construction of plasma torches wherein the plasma gas passes around the cathode and then flows concurrently with the arc to the anode. In most cases, the plasma gas travels in a spiral path to the anode. Some suggested structures are shown in U.S. Pat. Nos. 3,578,943 issued Mar. 19, 1969 to Schoumaker; 3,770,935 issued Nov. 6, 1973 to Tateno et al.; 4,670,290 issued Jun. 2, 1987 to Itoh et al.; or 4,855,563 issued Aug. 8, 1989 to Beresnev et al.
Tateno discloses a multiple arc system that incorporates a throttle aperture in the gas stream path and claim that the arc voltage may be increased to double that of conventional plasma jet generators in use at that time. Itoh et al describes a specific arrangement of a main and an auxiliary torch used in combination to form a hair pin arc which when formed extends from the cathode of the main to the cathode of the auxiliary torch to provide an extended arc length. An arc transfer system may be used to build the length of at least one of the arcs.
U.S. Pat. No. 3,140,380 issued Jul. 7, 1964 to Jensen and U.S. Pat. Nos. 4,982,067 and 5,144,110 issued Jan. 1, 1991 and Sep. 1, 1992 both to Marantz et al. show the use of concentric torches to generate a common plasma flow.
A preferred torch structure is shown in U.S. Pat. No. 5,008,511 issued Apr. 16, 1991 to Ross. In this torch, a plurality of individual torches are arranged around an axial passage through which the powder or other materials used in the plasma is introduced is thereby subjected to the plasma jets issuing from each of the torches. In this system, a cathode is provided within a chamber and has a cathode tip facing towards an anode. The plasma gasses are introduced and passed around the cathode, are heated by the arc between the anode and cathode, then pass out through a passage to contact with the powder material or the like.
It is well known that it is beneficial to operate a torch using as high a voltage as possible thereby minimize the amperage (A) required for a given power load, i.e. the range of amperage to voltage (V) i.e. (A/V) should be minimized and work is continuing see, New Plasma Spray Apparatus, Pashchenko and Saakov, Proceedings of the 7th National Thermal Spray Conference, 20-24 June 1994, Boston, Mass.
It is also known that several of the major factors influencing the ratio A/V in a given torch are;
a. the gas flow through the torch from the cathode to the anode, i.e. the higher the gas flow, the lower the ratio A/V, PA1 b. the composition of gas, PA1 c. the diameter of the arc; i.e. the smaller the arc diameter, the lower the ratio A/V, and PA1 d. the length of the arc; i.e. the longer the arc, the lower the ratio A/V.
In most torches, the passage extending from the cathode tip to the anode tapers to the smallest diameter at the anode, i.e. is generally or essentially the same cross-section for a significant portion of the distance between the cathode and the anode and then is tapered toward the gas outlet which is generally through the anode. Thus, the gas travelling through the passage leading to the anode outlet is not accelerated by the shape (cross sectional area) of the passage of the passage and its velocity remains substantially constant (except for the change in velocity due to the increase in temperature of the gases) until accelerated by the tapering of the passage toward the anode outlet. Thus in the length of the passage through which the arc passes the velocity is not controlled to confine the arc and extend its length before arcing or discharging to the anode.